In his words, here are the four questions you'll want to ask yourself to isolate said profitable skills:

What do you already pay for? We already pay people to do a lot of different things. Can you turn one of those things into your own online business? Examples: Clean your home, walk your pet, cook your meals, etc.

What skills do you have? Now, what do you know -- and know well? These are the skills you have that you're great at -- and people want to pay you to teach them. Examples: Fluency in a foreign language, programming knowledge, cooking skills, etc.

What do your friends say you're great at? I love this question. Not only can it be a nice little ego boost -- but it can also be incredibly revealing. Examples: Workout routines, relationship advice, great fashion sense, etc.

What do you do on a Saturday morning?What do you do on a Saturday morning before everyone else is awake? This can be incredibly revealing to what you're passionate about and what you like to spend your time on. Examples: Browsing fashion websites, working on your car, reading fitness subreddits, etc.

"Spend about 10 -- 20 minutes now writing down five answers for each of the four questions above," he writes. "Once you're done, congratulations -- you now have 20 potential business ideas that you can grow into a flourishing side hustle."

"What's the assumption behind it?" he asked. "The assumption is 'I'm waiting for a magical idea to fall down from the sky.' And that's not how it works. If you want to start a business, you find an idea. And it's not the perfect idea; it's one of many."

Once you start generating potential ideas, then you can test them to see if they're any good, and if they will make any money. But, Sethi says, just because you haven't been struck with inspiration doesn't mean you can't start a business.